The nuclear DNA content of 23 selachian species (10 Batoidea, 11 Galeomorphii, and 2 Squalomorphii) was histophotometrically studied. Their genome sizes range from 7.5 pg/N in Rajafillae (Batoidea) to 34.1 pg/N in Oxynotus centrina (Squalomorphii).
Results show slight differences in the pattern of quantitative variations between the superorders Batoidea and Galeomorphii; Squalomorphii preserve their peculiar wide interspecific variability at the intrafamilial level, with values sited between 13.1 and 34.1 pg/N.
In 21 species also the DNA base composition was determined by means of DAPI. The study shows that in the species examined the DAPI positive fraction varies from a minimum of 27.7~o in Oxynotus centrina, which possesses the largest genome size among all the Selachians studied, to a maximum of 72.5~o in Carcharhinus limbatus. As a whole the data show an inverse correlation between the DNA content and the DAPI positive fraction, a condition common to all cold-blooded vertebrates.
The low percentage of DAPI positive DNA found in Oxynotus centrina could be attributable to a lower stainability by the fluorochrome caused by a higher chromatin condensation in the erythrocytes.
The validity of the DAPI method was verified by comparison with the biochemical assay according to the thermal denaturation method in 6 selachian species.